A common technique for manufacturing printed circuit boards is to mount components directly on the surface of the printed boards by soldering. This has all but replaced the technique of securing components to a board by inserting wire leads from the components through drilled holes in the board, bending the leads over, then soldering them to conductive paths on the backside of the board.
In the surface mount technology, the surface of the board contains a multiplicity of pads, usually copper, to which components are directly soldered. The pads are connected by appropriate conductor paths in or on the board. Solder, in the form of thick, viscus paste is made of finely ground solder particles carried in a flux. The solder paste is applied only to the pads prior to the components being applied, which is then followed by the actual soldering. The solder is applied to the pads through a stencil which is superposed on the board with holes in the stencil corresponding in size and registered with the pads.
The boards are received, one by one, beneath the fixed horizontal stencil in a table adjustable on X and Y axes. The solder paste is wiped across the stencil and, thus, through the holes onto the pads.
There are problems common to conventional screen printers which are used in the stencil printing process. One problem relates to solder paste displacement. Solder is deposited on one edge of the stencil at the beginning of a wiping stroke and is wiped across the stencil. If the paste is displaced laterally of the stencil and it is not used for the next print, it will dry up and become worthless. This requires extra cleaning. Overcoming the problem requires constant operator attention, requiring him to continuously move the paste back to the leading edge of the print area. This is not only time consuming, but it is hazardous.
There are offset errors commonly encountered when using conventional printers. They are particularly noticeable when dealing with tightly toleranced printed circuit boards, such as those incorporating twenty to twenty-five mil pitch devices. Many screen printers employed in the stencil printing process have squeegees which spread the solder across the surface of the stencil. At the end of a wiping stroke, the squeegee is automatically lifted off the stencil and moved over the deposited ridge of solder paste and then lowered behind the ridge to repeat a wiping stroke in the opposite direction. This technique has been referred to as "hop over". While theoretically advantageous in actual production, sometimes as much as 70% of the paste will stick to the squeegee and, thus, end up on the opposite side of the squeegee for the return wiping stroke.
Another problem encountered in the process is for the squeegee blade to gradually displace the solder paste laterally of its direction of movement causing it to pile up outside the actual print area. While the "hop over" technique has been employed with some success, eventually there is not enough paste in front from the squeegee blade to produce a satisfactory deposition. At that time, the operator will have to physically move the paste by hand back into the print area. If he neglects to do so, the paste which is outside the print area and no longer being wiped by the squeegee, produces a poor deposition of paste onto the board and dries up and becomes useless.
One attempt to solve this problem was to employ printers with two squeegees that operate independently. The solder paste is located on the stencil between the squeegee and is wiped first in one direction and then in the opposite direction. This solution has created a new problem because printing with two squeegees can result in two different types of print. For example, sometimes the edges are not equally straight.
Another problem is stencil offset. The stencil itself is usually held in a frame by a border of tensioned polyester. During the wiping action of one squeegee, the stencil can be offset in the direction of wipe due to stretching of the polyester, thus, offsetting the stencil holes relative to the pads with which they are intended to register. This is referred to as "screen stretch". It is caused by the friction between the squeegee and the stencil applying stretching action to the polyester border.
Depending upon the width of the polyester border around the actual stencil foil, the length of the squeegee, and the amount of squeegee pressure against the stencil, the stencil offset from the pads could be from 0.002 to 0.005 inches in either direction, thus, totaling a four to ten mil total offset. Since this is a repetitious forward and backward offset, the stencil is stretched in two directions and not correctable.
In many instances, offset is not a problem because the solder paste is attracted back to the pads during the reflow or heating process as long as a 50 mil pitch or larger components are being used. However, many printed circuit boards require much higher tolerances, such as those incorporating 20 to 25 mil pitches. In these instances, the shifting of the image during printing is unacceptable. Therefore, it is necessary to eliminate all possible offset errors, which is a primary objective of the present invention.